Limit stop servocontrol system

ABSTRACT

Unidirectional feedback around a servoamplifier clamps the gain of the amplifier when the position feedback potential indicates the approaching of a stop. The clamping is in response to an amplifier output potential of a polarity that drives the servoed device into the stop, and is inoperative when the amplifier is of a polarity to drive the servoed device away from the stop. Full gain is therefore available when driving away from the stop. Increased accuracy is achieved, eliminating all position loop errors, by utilizing position feedback potential as a summing potential to control the operation of the clamping loop.

United States Patent Inventor Bertram F.Kupersmith [56] References CitedBloomfield, Conn. UNITED STATES PATENTS QY J 1 2: 1971 2,833,980 5/1968Hedgcocketal...... 318/626 t d 1,972 2,835,857 5/1958 Mosesetal. 318/6261 3,108,214 10/1963 wilkcl'SOn..... 318/158 Ass1gnee UnitedAircraitCorporation 3,197,684 6/1965 Rrege 318/626 Eastl-lartiord, Conn.

emanation muons No 3,201,675 8/1965 Curranetal 318/635 PP 3,403,3109/1968 Davidoff 318/626 794,625, Jan. 28, 1969, now abandoned. Thisapplication Mar. 17, 1971, Ser. No. 125,404

Primary Examiner-T. E. Lynch ABSTRACT: Unidirectional feedback around aservoamplifi- LIMIT STOP SERVOCONTROL SYSTEM er clamps the gain of theamplifier when the position feedback 2 Claims, 1 Drawing Fig. potentialindicates the approaching of a stop. The clamping is U 8 Cl 318/626 inresponse to an amplifier output potential of a polarity that 318/635,drives the servoed device into the stop and is inoperative Int Cl G0585/00 when the amplifier is of a polarity to drive the servoed deviceFieid 318/626 away from the stop. Full gain is therefore available whendriv- 635 158, g y from the stop Increased accuracy is achieved .2 1.eliminating all position loop errors, by utilizing position feedbackpotential as a summing potential to control the operation of theclamping loop.

7 Jae 1/050 anti R ff /fl .oawcg id /Z flaJ/r/av 4 ['[F'if/VCE' (7L) Val7,466 m Z M000! 6705 4C M70 'K Z? F aleasloea PAIENIEU Jag 4312 LIMITSTOP SERVOCONTROL SYSTEM This application is a continuation ofapplication Ser. No. 794,625 filed Jan. 28, I969, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of Invention This invention relatesto servo loop control systems, and more particularly to a unidirectionalservo limiter.

2. Description of the Prior Art In many servo loops known in the priorart, a position reference signal may be applied which calls for aposition that is past a mechanical stop or other physical limitation onthe position of the servoed device. If this occurs, the servomotor willremain operative, driving the device against the stop. This will causeit to dissipate full stall power for as long as the condition remains.In many instances, especially with high power servomotors, this mayresult in excessive overheating which in turn detrimentally affects thereliability and overall life of the motor. In order to overcome this,some devices known to the art have provided servo gain limitationswhenever positions are sensed near a stop. However, such limitations areoperative when driving away from the stop as well as when driving intothe stop. Thus, there is a significant loss in response speed of thesystem as a result of stop limitations.

SUMMARY OF INVENTION An object of the present invention is to provide aneffective limit to the position reference signal to a point very closeto a stop in a servosystem, with a tight tolerance under a wide varietyof power supply and temperature conditions, and with no loss inperformance.

According to the present invention, a feedback circuit is provided in aservoamplifier, the circuit being effective to substantially clamp theoutput of the amplifier to a voltage corresponding to a given positionreference voltage in response to a potential indicative of approaching astop. The feedback circuit is operative only in response to amplifieroutput potentials of a polarity tending to drive the servoed devicetoward the stop, and is inoperative in response to potentials tending todrive the servoed device away from the stop.

The present invention provides a relatively cheap and reliablecapability for servo limiting in the area of the stop, without loss ofsystem response characteristics when driving away from a stop. Thedevice is reasonably accurate under a wide variety of temperatures andpower supply variations, and may be implemented in simple technology.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE herein is a schematicblock diagram of a preferred embodiment of the invention, implemented tooperate in two polarities.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the figure, ina preferred embodiment, a servoed device is driven by' an AC motor 12which responds to a modulator 14 in dependence upon a DC potentialapplied thereto by an amplifier 16. The amplifier in turn responds tosignals provided by a feedback resistor 18, an input control signalthrough a resistor 20, and a position feedback signal through a resistor22, all of which are joined at a summing point 24 and applied to anegative input 26 of the amplifier 16. In response to a negative signalat the input 26, a positive signal is provided at an output 28 of theamplifier 16. For exemplary purposes herein, it is assumed that thepositive signal at the output 28 will cause the AC motor 12 to drive theservoed device 10 toward a given stop. This given stop is assumed to bein a direction equivalent to the top (most positive end) of a feedbackpotentiometer 30, the wiper of which is connected to resistor 22 so asto provide actual position information into the servo loop. This ismechanically driven by the servoed device 10. The resistor is connectedto a position reference voltage control 32 which may be, for instance,an

operator controlled device that indicates the desired position of theservoed device 10 by providing a suitable potential, such as may simplybe achieved by means of a voltage source and a potentiometer. Thefeedback loop including the resistor 18 incorporates a demodulator 34 toconvert an alternatingcurrent potential to a direct current potentialfor feedback input to the amplifier 16.

The system described thus far conforms to a typical type ofservosystemknown to the prior art. A description of the servo limitingin accordance with the present invention follows.

For exemplary purposes, consider the situation when the servoed deviceis driving toward the top of the potentiometer 30, and will reach a stopwhen the wiper of the potentiometer 30 nears the most positive endthereof. The circuitry now to be described is used to limit theoperation of the servosystem as the servoed device approaches that stop.The wiper of the potentiometer 30 is connected to a resistor 36, throughanother potentiometer 38 and a resistor 40 to a negative supply. Thus,there is a voltage divider from the positive source through the feedbackpotentiometer 30, and the resistances 36, 38 and 40. The base of anNPN-transistor 42 is connected to the wiper of the potentiometer 38,which wiper may be adjusted in order to adjust the point of operation ofthe transistor 42 in comparison with the position of the wiper on thepotentiometer 30, and the actual voltage and resistance values used inthe system. The emitter 44 of the transistor 42 is connected to thesumming point 24, and the collector 46 of the transistor 42 is connectedthrough a diode 48 to the output 28 of the amplifier 16. The diode 48 ispoled to be conductive when the output of the amplifier 16 is positive.Thus the transistor 42 can in turn operate with gain only duringpositive outputs of the amplifier 16. Assuming that the resistances 36,38 and 40, and the adjustment of the wiper of the potentiometer 38, areall selected so as to give proper operation, when the wiper of thepotentiometer 30 reaches a given point near the stop when limitingaction is desired, the transistor 42 will conduct thus connecting theoutput of the amplifier 16 at point 28 to the summing network 24 at theinput 26 of the amplifier 16. This effectively clamps the amplifier atthat potential, and prevents any further driving of the servoed device.Thus, the device is electrically limited before reaching its mechanicalstop, and driving against the stop is avoided.

Now consider the case where the servoed device 10 is driven toward astop which is equivalent to the most negative part of the potentiometer30. To provide servo limiting in this direction, the resistor 36 is alsoconnected through a potentiometer 50 and a resistor 52 to a positivesource. The wiper of the potentiometer 50 is connected to the base of aPNP transistor 54, the emitter 56 of which is connected to the summingjunction 24. The collector 58 of the transistor 54 is connected througha diode 60 to the output 28 of the amplifier 16. Only when the output ofthe amplifier 16 is negative will the diode 60 conduct, and therefor thetransistor 54 will be operative to clamp the amplifier 16 only in thecase where the most negative end of the potentiometer 30 is reached inresponse to a negative output signal.

Note that in the first instance, when the servoed device is drivingtoward the most positive portion of the feedback potentiometer 30, inthe event that the position reference voltage control 32 is readjustedso as to provide a positive voltage and tend todrive the servoed device10 towards a position which corresponds to the most negative position ofthe wiper on the feedback potentiometer 30, then the output of theamplifier 16 becomes negative so diode 48 no longer conducts and thelimiting action of feedback through transistor 42 does not affectdriving the servoed device away from the stop. On the other hand, assoon as the negative output from amplifier 16 causes diode 48 tobereversed biased, the diode 60 is forwardly biased. But since thefeedback potentiometer 30 has been set near the positive end, transistor54 does not conduct so there is no limiting action in this case unlessthe device is driven all the way toward the negative end of the feedbackpotentiometer 30.

Notice that the transistors 42, 54 are controlled directly by potentialsdeveloped by the feedback potentiometer, and therefore errors'inoperating the clamping of the servosystem in accordance with the presentinvention are minimized. Similarly, regardless of minor differences inthe power supply potentials which may occur and of resistance variationsas a result of temperature changes; the direct operation by utilizing aclamping feedback loop which is directly controlled by the feedbackpotentiometer provide relatively accurate operation.

Although the invention has been shown and described with respect to apreferred embodiment thereof, it should be understood by those skilledin the art that various changes and omissions in the form and detailthereof may be made therein without departing from the spirit and thescope of the invention.

Having thus described a typical embodiment of my invention, that which Iclaim as new and desire to secure by Letters Patent of the Unites Statesis:

1. A servoamplifier system adapted to drive a servoed device toward andaway from at least one limit stop in response to a desired positionreference input signal and an actual position feedback signal indicativeof the position of the servoed device, said system adapted to limit theoutput signal of an amplifier in the amplifier system as the servoeddevice approaches the stop, yet not limit the output signal of theamplifier when driving the servoed device away from the stop, saidsystem comprising:

an amplifier circuit having an input and an output out of phase withsaid input;

a summing network connected to said input;

a servoed device driven 'in response to the output signal of saidamplifier circuit and having at least one position limit stop;

means connected to said summing network for providing a desired positionreference signal thereto;

position feedback signal means controlled by said servoed device todevelop an actual position feedback signal indicative of the actualposition of said servoed device;

means responsive to said position feedback signal means and connected tosaid summing network for coupling said actual position feedback signalthereto;

limit means responsive to said position feedback signal means fordeveloping a limit stop signal indicative of the relative position ofsaid servoed device with respect to said limit stop;

and limit stop feedback means connected directly between said output andsaid summing network, said limit stop feedback means having an operativestate and a nonoperative state, said limit stop feedback means when inits operative state providing unilateral conduction between saidamplifier circuit output and said input summing network, thereby toclamp said amplifier circuit with negative feedback, said limit stopfeedback means poled so as to conduct output signals from said output tosaid summing network only of a polarity tending to drive said servoeddevice toward said limit stop, and not of a polarity tending to drivesaid servoed device away from said limit stop, said limit stop feedbackmeans connected to said limit means and rendered operative by a limitstop signal of a given magnitude.

2. The servoamplifier system according to claim 1 wherein said servoeddevice has a second limit stop, and additionally comprising:

second limit means responsive to said position feedback signal means fordeveloping a second limit stop signal in dicative of the relativeposition of said servoed device with respect to said second limit stop;

and second limit stop feedback means connected directly between saidoutput and said summing network, said second limit stop feedback meanshaving an operative state and a nonoperative state, said second limitstop feedback means, when in its operative state providing unilateralconduction between said amplifier circuit output and said input summingnetwork, thereby to clamp said amplifier circuit with negative feedback,said second limit stop feedback means poled so as to conduct outputsignals from said output to said summing network only of a polaritytending to drive said servoed device toward said second limit stop, andnot of a polarity tending to drive said servoed device away from saidsecond limit stop, said second limit stop feedback means connected tosaid second limit means and rendered operative by a second limit stopsignal of a given magnitude.

1. A servoamplifier system adapted to drive a servoed device toward andaway from at least one limit stop in response to a desired positionreference input signal and an actual position feedback signal indicativeof the position of the servoed device, said system adapted to limit theoutput signal of an amplifier in the amplifier system as the servoeddevice approaches the stop, yet not limit the output signal of theamplifier when driving the servoed device away from the stop, saidsystem comprising: an amplifier circuit having an input and an outputout of phase with said input; a summing network connected to said input;a servoed device driven in response to the output signal of saidamplifier circuit and having at least one position limit stop; meansconnected to said summing network for providing a desired positionreference signal thereto; position feedback signal means controlled bysaid servoed device to develop an actual position feedback signalindicative of the actual position of said servoed device; meansresponsive to said position feedback signal means and connected to saidsumming network for coupling said actual position feedback signalthereto; limit means responsive to said position feedback signal meansfor developing a limit stop signal indicative of the relative positionof said servoed device with respect to said limit stop; and limit stopfeedback means connected directly between said output and said summingnetwork, said limit stop feedback means having an operative state and anonoperative statE, said limit stop feedback means when in its operativestate providing unilateral conduction between said amplifier circuitoutput and said input summing network, thereby to clamp said amplifiercircuit with negative feedback, said limit stop feedback means poled soas to conduct output signals from said output to said summing networkonly of a polarity tending to drive said servoed device toward saidlimit stop, and not of a polarity tending to drive said servoed deviceaway from said limit stop, said limit stop feedback means connected tosaid limit means and rendered operative by a limit stop signal of agiven magnitude.
 2. The servoamplifier system according to claim 1wherein said servoed device has a second limit stop, and additionallycomprising: second limit means responsive to said position feedbacksignal means for developing a second limit stop signal indicative of therelative position of said servoed device with respect to said secondlimit stop; and second limit stop feedback means connected directlybetween said output and said summing network, said second limit stopfeedback means having an operative state and a nonoperative state, saidsecond limit stop feedback means, when in its operative state providingunilateral conduction between said amplifier circuit output and saidinput summing network, thereby to clamp said amplifier circuit withnegative feedback, said second limit stop feedback means poled so as toconduct output signals from said output to said summing network only ofa polarity tending to drive said servoed device toward said second limitstop, and not of a polarity tending to drive said servoed device awayfrom said second limit stop, said second limit stop feedback meansconnected to said second limit means and rendered operative by a secondlimit stop signal of a given magnitude.